It really does vary. Going down on her is a huge turn on for me, so if it's been a few says and we start things off with me going down on her it'll be maybe 5 minutes before I reach the edge. If we've recently done it, or it's round 2, or if foreplay was short I can keep going for 45 minutes.
Of course it also depends on how long I want to last. I can probably finish in about 5-10 minutes if we're just having a quickie.